A MEETING WITH THE UNIVERSE

Chapter 4-7

The Galaxies: Islands of Stars

Beyond the Milky Way

Stars are not scattered uniformly
across the seas of space. They are
collected into huge disks, spirals, and
globular forms that may contain billions
of stars. These may be more than
100,000 light years across, but they
are often millions of light years apart.
Hence, they have occasionally been
called "island universes".

Astronomy before the Space Age
was mostly concerned with stars and
star clusters. It was not until 1924
that the existence of galaxies beyond
our own Milky Way was firmly established
by the then-new 100-inch telescope
at Mount Wilson. Once scientists
were convinced that those dim,
fuzzy patches in the sky actually contained
hundreds of billions of stars, a
new astronomy was started.
It was quickly realized that galaxies
come in different shapes and
sizes. The most numerous ones are
dwarf galaxies like the Magellanic
Clouds, two satellites of our own galaxy.
Dwarf galaxies are small and
often irregular, but even they can
contain a million to a billion stars. At
the other end of the scale, some of the
very biggest galaxies are elliptical in
shape, ranging from almost spherical
to very elongated like a football. Many
people, scientists and nonscientists
alike, feel that the most beautiful galaxies
are the spirals, such as the famous
Andromeda galaxy, whose mass
amounts to half a trillion suns. Our
own galaxy, the Milky Way, is a comparable
spiral, although the details of
its structure are hard to determine
from within. Besides the normal spiral
galaxies such as Andromeda, there
are spiral galaxies with a bar across
the middle, the spiral arms trailing
like pennants at the end of the bar.

As ground-based investigations
of galaxies proceeded, it was soon
learned that far-off galaxies are receding,
and that the more distant the
galaxy, the faster it is moving away
from us. This discovery was a major
achievement of the 100-inch telescope
at Mount Wilson, and it provided
the impetus for the Big Bang
theory of formation of the universe.
Studies of the detailed appearance of
galaxies also revealed several new
types. Among these are the Seyfert
galaxies (named for the American astronomer
Carl Seyfert), which have
very bright central nuclei. The spectra
of these nuclei reveal the presence
of hot gases in rapid motion.
Other remarkable aspects of galaxies
were found when radio telescopes
were developed. After some
galaxies were found to emit intense
radio waves, more detailed observations
of these radio galaxies showed
a characteristic double-lobe structure
of the radio-emitting regions. These
lobes typically are located far beyond
the visible structure of the galaxy.

The amount of energy required to
produce the lobes is immense, some
times equal to the output of a galaxy
over its entire life. Later it was found
that at the centers of some radio gal
axies there are very intense radio
emitting cores which could not be spatially
resolved even with the biggest
radio telescopes. The character of the
radio waves showed that these
sources must be very small - in some
cases as small as the solar system - and
yet somehow they produce enormous amounts of energy. This has
been further corroborated by infrared
measurements.

Another unusual galaxy, known
as Centaurus A, has been the subject
of some exciting space observations.
Centaurus A has a typical double-lobe
radio source, but in addition there is
an inner pair of lobes, spaced along
the same axis as the outer lobes.
Within these inner lobes there is yet
another, unresolved source of radio
emission. In addition, Centaurus A
produces a tremendous amount of
high-energy radiation. The gamma
ray telescope on HEAO-I revealed
that Centaurus A emits gamma rays
with energies up to 1 million electron
volts. Another experiment detected
gamma rays from Centaurus
A with even higher energies of 100
billion electron volts. The latter, very
high-energy gamma rays produce
showers of electrons in the Earth's
atmosphere.

X-rays are also emitted from the
inner radio lobes of Centaurus A. In
addition, there is ajet, visible to X-ray
telescopes, that extends from the innermost
core to the northern inner
lobe. The existence of thejet indicates
that the lobes are constantly
resupplied with energy from the
active, but still mysterious
nucleus of the galaxy.

Normal Galaxies

The nearest neighbors to our own
Milky Way galaxy are the Magellanic
Clouds, about 150,000 and 190,000
light years away. To observers in the
southern hemisphere they resemble
luminous clouds several times the
size of the full Moon, but the Magellanic
Clouds were not well explored
until recently, when large telescopes
were built in the Southern Hemisphere
and airborne telescopes conducted
far-infrared observations. In
the Magellanic Clouds, our satellite
observatories are finding the kinds
of X-ray sources that we earlier discovered
in our own galaxy: binary
stars, supernova remnants, and
others. In addition, the most intense
gamma ray burst yet observed, the
event of March 5,1979, apparently
came from the Large Magellanic
Cloud.

Gamma ray bursts are a relatively
new discovery from orbiting satellites.
Many objects in the universe emit
gamma rays fairly steadily, but these
sudden eruptions of gamma rays from
a single point typically last a few seconds
to tens of seconds, during which
they outshine the rest of the universe
in gamma rays. We think that the
sources of typical gamma ray bursts
are somewhere in our own galaxy,
perhaps as close as 20 to 50 light years
from Earth. But if the intense burst
on March 5, 1979 came from as far
away as the Large Magellanic Cloud,
being the release of energy must have been
truly enormous for the event to seem
so bright at a distance of 150,000 light
years. The direction of the source of
this burst has been determined with
great precision by combining measurements
from nine different spacecraft,
and it lies near the heart of a
supernova remnant in the Large Magellanic
Cloud. Perhaps an unusual
neutron star left behind by the supernova
explosion was the actual
source of the gamma rays.

Our next neighbor galaxy is the
Great Spiral Galaxy in Andromeda,
the most distant object visible to the
naked eye. It is as large as or larger
than our galaxy and is about 2 million
light years distant, yet emits only
about one-tenth as much energy in
the form of infrared radiation as our
galaxy. X-ray images taken with a
telescope on the HEAO-2 satellite
revealed more than 70 X-ray sources,
which appear to be binary stars and
super nova remnants in this neighbor
galaxy.

Deep in the heart of Andromeda.
X-ray emission marks the locations of powerful energy sources in the Andromeda
galaxy: Photographed in visible light, M31, the Great Spiral Galaxy in
Andromeda, resembles our own Milky Way as it might appear to a distant
observer. Two small elliptical galaxies are satellites of M31. (Copyright,
California Institute of Technology and Carnegie Institution of Washington.)

Deep in the heart of Andromeda (cont.)
Composite of two photographs made in
ultraviolet light from an Astrobee rocket in
August, 1980 emphasizes regions where hot
stars are present, notably in spiral arms.
Central bulge of the galaxy, largely consisting
of cooler stars, is less prominent than in
photo above. (Courtesy of R.C.Bohlin and
T.P.Stecher, Goddard Space Flight Center.)

Deep in the heart of Andromeda (cont.)
Short exposure on bright central
region of M31 reveals faint dust lanes threading
an aggregation of innumerable stars, one
seemingly indistinguishable from the other.
(Official U.S. Naval Observatory photograph)

Deep in the heart of Andromeda (cont.)
Seen in a new, way, M31 central
region was imaged in its own X-rays by an
instrument on the HEAO-2 satellite. Strong
sources of X-rays (bright spots) are few enough
to count, but more common near Andromeda
galaxy's center than in the nuclear region of
our own Milky Way. Observations made at
intervals show that the intensities of many
X-ray sources are changing.

We have just discovered that our
own Milky Way is surrounded by a
corona, a very thin atmosphere of hot
gas. The sources of the corona probably
are the bubbles of hot, thin gas
that pervade the interstellar space in
our galaxy. Some of the bubbles probably
expand to such enormous sizes
(thousands of light years in diameter)
that they must actually break out of
the disk of the galaxy altogether, in
jecting hot gas into intergalactic
space. Much of the hot gas remains
bound to the galaxy by gravitation
and thus forms the invisible corona at
a temperature of about 100,000° C.

Because the galactic corona produces
no visible light, it could not be seen
until the launch of the International
Utraviolet Explorer (IUE) satellite in
January, 1978. IUE found evidence of
the corona in the form of ultraviolet
absorption lines in the spectra of
bright blue stars in the Magellanic
Clouds. The measured Doppler shifts
of the lines proved that they are produced
by foreground gas around the
Milky Way, rather than in the Clouds.

The galactic corona.
Our Milky Way galaxy
is surrounded by a galactic corona of thin
hot gas. The corona was found when the
International Ultraviolet Explorer (lower left)
recorded the spectra of hot, bright stars in the
Large and Small Magellanic Clouds, two
neighbor galaxies of the Milky Way.
The spectra showed dark lines identified as
absorptions by gas in the corona. The gas rotates
along with the Milky Way, so it is not simply
a medium in which the galaxy is embedded.
Similar observations now reveal coronae
around each of the Magellanic Clouds and at
least on other, more distant galaxy.

Some coronal gas is not gravitationally
bound to our galaxy and must
expand into the space between the
galaxies. The scales of distance here
are immense. Galaxies are tens to
hundreds of thousands of light years
across, but the distance between galaxies
is typically a few million light
years, a million times the average distance
between stars in our galaxy.
Even if Voyager, with its great speed,
could escape our galaxy, it would take
30 billion years for it to reach the
Andromeda galaxy, much longer than
the present age of the universe!

In places where galaxies cluster
together to form a group millions of
light years across, the escaping hot
gas between the galaxies is revealed
by its X-ray emission. With the X-ray
telescopes of HEAO-2, we obtained
images of the gas in such clusters of
galaxies. We have even found clues to
the age of the gas. For example, in
clusters where the gas has only recently
emerged from its parent galaxies,
it is still clumped around them, as
revealed by the patchy appearance of
the X-ray images. In more evolved
clusters, the images are smooth,
showing a diffuse, centrally-peaked
distribution of hot gas. These differences
in the X-ray images of clusters
of galaxies are correlated with differences
in the types of galaxies in
volved. Spirals seem to be associated
with the patchy X-ray emission. Thus,
X-ray observations are providing
basic clues to the evolution of giant
systems of galaxies. We have only a
few tantalizing clues just now, but the
picture should become clearer after
the launch of the planned Advanced
X-ray Astrophysics Facility in the
late 1980s.